Natural gas is a clean fossil fuel of which the demand has skyrocketed globally and the resource development has been fiercely competed because it generates significantly smaller quantities of carbon dioxide per fuel mass during the combustion than coal and petroleum.
Natural gas that is produced from gas fields is used as fuel through transportation and storage processes after removing mostly sulfur, carbon dioxide, water and polymer hydrocarbon but methane.
Since the price of natural gas is mostly dependent upon the facility and operation costs of implementing the above processes in addition to the margin and interest, the most economical transportation and storage method is selected, considering various factors such as the size of the gas field and the distance to the consumer. The most typical marine transportation method is the LNG (liquefied natural gas) method, and the compressibility of LNG is about 600 when it is normal condition methane.
Nonetheless, the economic feasibility of the LNG method is restricted due to the cryogenic requirement of LNG, and thus the LNG method is applicable for gas fields with a certain scale or more (i.e., currently at least about 3 trillions of cubic feet).
In order for methane, which is the main component of natural gas, to exist stably as a liquid under normal pressure, the temperature needs to be −162 degrees Celsius or lower. Accordingly, metal materials used in the LNG facility that is exposed to cryogenic conditions need to include high concentrations of expensive nickel so as to minimize the brittleness. Moreover, due to a great difference in temperature between the inside and the outside during the transportation and storage processes, heat influx causes a large amount of BOG (boil off gas) to be generated.
In order to achieve economic feasibility of developing relatively small scale gas fields by overcoming these shortcomings and saving production costs of natural gas, GTS (gas to solid) technologies have been widely studied to transport/store natural gas using solid gas hydrate as storage medium. Particularly, in 1990, a Norwegian professor, named Prof. Gudmundsson, presented the self-preservation effect theory of hydrate to motivate many industrialized nations, such as Japan, to develop key technologies required for realizing commercial GTS methods.
Natural gas hydrate (NGH), which is crystal mixture in which natural gas molecules are collected within solid state lattices of hydrogen-bonding water molecules, has an external shape that is similar to ice and maintains its solid state stably if a pressure that is higher than a certain value is applied at a given temperature. In order for methane hydrate to stably exist thermodynamically under normal pressure; the temperatures needs to be −80 degrees Celsius or lower, but the self-preservation effect of delaying the decomposition of hydrate for several weeks is discovered when ice film is formed on the surface of a hydrate particle at temperatures of about −20 degrees Celsius.
The gas compressibility of NGH is about 170 (that is, about 170 cc of normal condition natural gas is stored in 1 cc of hydrate), which is disadvantageous than LNG, but the temperature condition for transportation and storage of NGH is more advantageous. Accordingly, it has been theoretically verified that the GTS method using NGH is an economically alternative option of the LNG method for small-to-medium scale gas fields.
The elemental technologies constituting the GTS method include the NGHP (natural gas hydrate pellet) production technology, which transforms natural gas to the pellet type of hydrate before transporting/storing natural gas, and the revaporizing technology, which recovers natural gas by decomposing the NGH afterwards.
The conventional device for revaporizing natural gas induces decomposition of NGH pellets, which have been charged into a storage tank that is also for transportation, by supplying heating water from the bottom of the tank at the location of consumption, discharges the water that is decomposed and the supplied hot water after it is cooled to an outside, and recovers the decomposed gas.
This method, however, has shortcomings that it is not possible to produce a large amount of high-pressure gas continuously and that it is not possible to use the residual gas that is remaining inside the tank.
Moreover, although the conventional continuous revaporizing technology reflects the basic concept of separating and recovering high-pressure gas generated by inputting NGH pellets into a revaporization reaction tub, which is heated by the circulating heating water, from the decomposed water, the conventional continuous revaporizing technology lacks specific details required for making a practical revaporizing device.